1. Field of the Invention
This invention relates to the field of sensing electromagnetic radiation, and more particularly to a method and apparatus for multiplexing signals from an array of electromagnetic radiation detectors.
2. Description of Related Art
Elemental infrared detectors are often used in conjunction with missiles and night vision systems to sense the presence of electromagnetic radiation having a wavelength of 1-15 .mu.m. These detectors often operate on the principle of photoconductivity, in which infrared radiation changes the electrical conductivity of the material upon which the radiation is incident. Because they are often most sensitive when operating at cryogenic temperatures, photoconductive infrared detectors such as those fabricated from mercury-cadmium-telluride are generally used with a cooling device known as a cryoengine which produces and maintains the necessary operating temperature.
While an array of elemental infrared detectors may be used in an elemental system in which the detectors sense the energy generated by an object space, elemental detectors may also be used in thermal imaging systems. In some real time thermal imaging systems such as forward looking infrared ("FLIR") imaging sensors, moving mirrors are used to scan radiation emitted by the object space across a linear array of elemental detectors. The temporal outputs of the detectors form a two-dimensional representation of the thermal emission from the object space.
To obtain electrical signals from an array of elemental photo-conductive infrared detectors, each elemental detector is generally associated with an individual output conductor. In addition, ground or return paths are also provided, and groups of elemental detectors are usually connected together as well as to common returns. Since the number of elemental detectors in a detector array can often exceed 150 detectors, the number of conductors required to deliver signals to and from the detector array often is greater than 150. The relatively large number of conductors required to be connected to the array tends to increase the amount of undesirable thermal energy which is delivered from the environment to the array through the conductors. Though it is possible to reduce the cross-section of the conductors to minimize the flow of thermal energy to the detectors, the reduction in cross-section is often accompanied by an increase in the resistance of the conductors. Since photo-conductive detectors often have low impedance, the high resistance of the conductors appearing in series with the detectors would cause noise and crosstalk problems. In addition, problems also exist with respect to connecting the relatively large number of conductors to the small closely spaced elemental detectors forming the array.
One method for reducing the number of conductors from elemental detector arrays is to use time division multiplexing in which the signals from two or more detectors are delivered over a common conductor during successive time intervals. However, the use of time division multiplexing often adversely influences the noise performance of the detectors which have signal levels in the order of microvolts. The circuits used to perform time division multiplexing generate noise as they are generally driven by control signals of one volt or greater and cannot be totally isolated from the signal path. The use of amplifiers to reduce the noise associated with time division multiplexing is not generally feasible as the amplifiers often increase the heat load of the system. In addition, while it is possible to use photovoltaic detector arrays with silicon readout arrays which require fewer conductors, it is difficult to manufacture photovoltaic detectors in large quantities which are able to operate in the most desirable spectral region (i.e., 8-12 .mu.m) at easily reached operating temperatures.